Playing at 100% effort all the time is not something anyone does, ever. Watch soccer (non-US: football) players. Are they going at 100%? Only if they have a scoring chance, or are defending a scoring chance, for the most part. Playing at full speed in soccer means you're basically sprinting, and who can sprint for 90 minutes (or even 45)?
Mental effort, or attention, is a mental version of physical exertion, and it requires the same kind of careful management. You can spend 100% of currently available attention all the time, but then you run out. Instead, players typically marshal that attention the same way they marshal their physical exertion: pay close attention when in a crucial scoring chance, pay less attention when well off the ball.
None of this is to say that improving your mental abilities isn't impossible; some of that comes with simply playing more and expending that full effort sometimes, just as with improving your cardiovascular endurance or your muscle strength. You can do mental exercises; some are going to be sport specific - search for your sport and "focus drills" or "mental toughness drills" - while some will be more general in nature.
For the most part I'd recommend starting with the sport-specific ones, as you need different things in different sports - Soccer and Tennis are very different, for example.
But the main thing I think here is that you shouldn't consider yourself a bad person because you can't focus 100% all of the time - you're no different from anyone else there. How many Grand Slams would Serena Williams have if she could give 100% every play, every day? LeBron James can't give 100% on every play, either, and while he gets a ton of flak when he does take a play off, it hasn't hurt him too much. Play at the intensity you're able to, and turn it up to 100% in those times where you need it - during that scoring chance or that critical defensive chance, when you need the extra energy.